Evolution of atmospheric carbon dioxide concentration at different temporal scales recorded in a tall forest

Understanding the temporal evolution of CO2 over specific ecosystems can provide additional insight into atmospheric CO2 variability. From 2003 to 2010, CO2 ambient concentrations in an old-growth temperate forest were measured continuously by using an infrared analyzer. The results show that daily cycle and seasonal variation of CO2 were mostly dominated by vegetation activity, whereas the weak turbulent mixing at night contributed frequently to CO2 build-up below the canopy. As a result, CO2 concentrations changed considerably during daytime and nighttime in the forest stand. The monthly means exhibited a seasonal variation characterized by a primary peak in December and a minimum during July to August, with an average seasonal amplitude (the difference of minimum and maximum monthly means) of 60.5 ppmv. By fitting a harmonic model to the monthly means, linear increase trends of 1.7 ppmv yr−1 in the annual mean CO2 concentrations and 0.58 ppmv yr−1 in the amplitude of the seasonal cycle were estimated. The upward trend of annual mean CO2 concentration may be attributed to the dramatic global increase in CO2 emissions during the study period. This study also found that the stratification of CO2 concentrations below the canopy during nighttime was easily disrupted after sunrise, which suggests that forest floor plants are not, as previous assumed, exposed to significantly greater CO2 concentrations than the normal atmospheric concentrations.